The manufacture of fibres and films from cellulose by the viscose process has been known for more than a hundred years. Even today, almost all cellulose-based fibres are manufactured by the viscose method. It is a known method, by which various properties of the final product are achieved by varying the material and process parameters. However, the viscose method involves significant drawbacks: the preparation of the spinning solution includes laborious work stages, the carbon disulphide used for the dissolution is toxic, inflammable and combustible, and it is difficult to recover. Furthermore, some of the carbon disulphide is decomposed to hydrogen sulphide, which is also toxic and explosive. In addition, the viscose solution is an unstable product, whereby it cannot be stored as an intermediate product, but all the steps of the manufacture must be taken without a delay from the beginning to the end, keeping the mass at a low temperature.
Several attempts are known to replace the viscose method with a more ecological method. The most promising one has been the conversion of cellulose to cellulose carbamate by means of urea (see, for example, D. Klemm et al., Comprehensive Cellulose Chemistry, Wiley-VCH 1998). In spite of its obvious advantages and several known attempts, this method has, however, remained on the laboratory scale. Reasons have included problems in the homogeneity of the product, the recovery and residues of organic auxiliary agents (e.g. hydrocarbon) and/or solvents (normally ammonia) used, the properties of the final products (primarily fibres), which have been not more than satisfactory, and the operation costs of the methods developed.
Known attempts to provide a method for manufacturing cellulose carbamate have been based on the soaking of pulp sheets in an alkaline solution (mercerization), which has, in some cases, included an addition of ammonia and/or other solvents or accelerators. After the mercerization, the pulp, partly dried by compressing, is treated in a urea solution, which may include an addition of an alkalizing agent, normally also ammonia and possible solvents or salts. Finally, the reaction between urea and the pulp is carried out in an oven at a temperature of about 130° C. The methods have required the best viscose cellulose whose DP level has been reduced, for example, by long-term curing in a mercerization solution or by irradiation in advance. Examples of the above-described processes are presented in patents FI 61033, EP 0 402 606 and WO 00/08060.
One of the first attempts to manufacture cellulose carbamate is presented in U.S. Pat. No. 2,134,825. It uses the aqueous solution of urea and sodium hydroxide, with which the pulp sheets are first impregnated. After the impregnation, settling and compression, the mass is dried and heated in the oven to achieve a reaction between the cellulose and urea. The patent presents a number of chemicals to improve the absorption and to reduce the gelling tendency of the solution. This patent also presents the use of hydrogen peroxide for the purpose of reducing the viscosity of the solution. However, pulps manufactured on the basis of the patent have been only partly soluble in such a way that a large quantity of unreacted fibres is left in the solution, jamming the spinning nozzle. This is probably due to the unevenness of the substitution.
In all known methods for manufacturing cellulose carbamate, an alkaline solution (aqueous sodium hydroxide) is used for activating (swelling) the pulp, as in conventional mercerization of pulp. An exception to this, U.S. Pat. No. 2,134,825 experiments the use of hydrogen peroxide with and without sodium hydroxide to activate the pulp for the purpose of reducing the viscosity of the solution.
Cellulose carbamate is alkali soluble at a substitution degree of 0.2 to 0.3. The formation of cellulose carbamate begins when the mixture of cellulose and urea is heated to a temperature exceeding the melting point of the latter (133° C.). When heated, urea is decomposed to isocyanic acid and ammonia according to the following reaction formula:NH2—CO—NH2→HN═C═O+NH3 
Isocyanic acid is very reactive and it forms carbamates with the hydroxy groups of cellulose as follows:Cell-OH+H—N═O→Cell-O—CO—NH2 
Possible side reactions include the reaction of urea and isocyanic acid to a biuret, or the formation of cyanuric acid and other polymerization products of isocyanic acid.